An approach of assessing reliability for multiple NC machine tools with minimal repair based on power law process (PLP) was proposed, and a PLP function for failure times was built. Trend test and renewal test were performed on the failure data to assure the application of power law process was valid. CBT (common beta test) and CLT (common lambda test) two-step-test was developed to check the homogeneity of multiple machines in assessing. Point and confidence interval estimators were given by maximum likelihood method and Fish information matrix. Point estimates of reliability indexes were also derived with their confidence bounds offered through Delta method. The Cramér-von Mises method was finally adopted to do the goodness-fit-test. Real example analysis demonstrates that PLP can give both instantaneous and cumulative reliability properties for NC machine tools, CBT&CLT test is capable to verify the homogeneity of multiple machines adequately, hence the results are more practical.